The present invention relates to the determination of the location of a mobile radio-frequency transceiver operating within the operational domain of a wireless communications network. In particular, the mobile units of primary interest are cellular telephones, personal digital assistants, wireless-equipped laptop computers, and other similar devices equipped with wireless transceivers for normal operation under a “cellularized” telephone system, such as one based on the Global System for Mobile communications (GSM). The location-determination technology described herein optimally integrates GPS data together with infrastructure data and collateral data, for enhanced accuracy and robust effectiveness under conditions that could degrade results individually achievable under the distinct approaches.
As realized and noted in extensive prior art, the ability to routinely, reliably, and rapidly locate cellular wireless communications devices has the potential to provide significant public benefit in public safety and convenience and in commercial productivity. Many systems have been described for the determination of the locations of such communications devices through the implementation of an infrastructure of location-determination facilities in affiliation with the communications system infrastructure. Examples of such infrastructure-based (or network-based) systems for the determination of locations for wireless mobile units are found in Stilp, et al., U.S. Pat. No. 5,327,144; Stilp, et al., U.S. Pat. No. 5,608,410; Kennedy, et al., U.S. Pat. No. 5,317,323; Maloney, et al., U.S. Pat. No. 4,728,959; and related art. The use of collateral information to enhance and even enable location determination in further applications of such infrastructure-based systems was introduced in Maloney, et al., U.S. Pat. No. 5,959,580; and further extended in Maloney, et al., U.S. Pat. Nos. 6,108,555 and 6,119,013. These and related following descriptions of the prior art for infrastructure-based location determination systems enable robust and effective location-determination performance when adequate measurement data can be derived or are otherwise available.
A dominant benefit in the use of infrastructure-based location systems is the applicability of the technology for the localization of any and all types of mobile wireless communications units. The infrastructure technology establishes the facility to locate the mobile units through the measurement of location-related signal characteristics inherent in the normal communications-band transmissions. Thus legacy phone models as well as newly emerging wireless communications units can all be equally served with the location facilities. The mobile units need only employ the standard wireless communications system signal formats and protocols, and do not require any special, localization-specific modification to support the location capability.
A difficulty that accrues in the application of the infrastructure-based systems occurs with their use in sparsely populated, rural environments. In these environments, the economic constraints of the underemployed communications facilities only support the deployment of the communications facilities at cellular land stations that are significantly distant from each other. With the similarly sparse deployment of location-determination facilities among the available cell sites, the accuracy of the calculated locations is degraded relative to that achievable in urban and suburban environments. In the urban and suburban environments, the spatial densities of the communications cell stations are high in order to service the demand for the communications traffic without routinely exceeding the capacities of the individual cells. With the similar deployment of location-system facilities at cell stations in comparatively close proximity to each other, the location determinations are derived with significantly higher quantities of contributing measurements, extracted at sensing locations with significantly higher signal strengths, providing significantly better cumulative precision for the location evaluations. The sparse deployment densities of the infrastructure equipment in rural environments challenge the accuracy capabilities for the infrastructure-based location determination systems.
Location-determination systems based upon the use of the U.S. government's Global Positioning System (GPS) are very accurate when the GPS receiver has reception access to an open sky. The constellation of GPS satellites transmitting from over head provides the signals from which the GPS receiver can determine its location. In rural domains where the view of the sky is open, exemplary GPS accuracy is routinely achievable.
Significant prior art is available for the use of GPS receivers embedded with mobile wireless communications transceivers to locate the mobile units. Example descriptions of such approaches are included in, e.g., U.S. Pat. No. 4,445,118, Apr. 24, 1984, “Navigation System and Method”; and U.S. Pat. No. 6,538,600 B1, Mar. 25, 2003, “Wireless Assisted GPS Using a Reference Location.” The accuracy of the GPS devices is superb when an adequate number of relatively undistorted satellite signals is able to be received at sufficient signal strength.
Several difficulties are inherent in the use of GPS augmentations for determining locations for wireless communications units. The GPS facilities in the mobile units are distinct from the communications facilities, and hence only phone models that incorporate the additional hardware functionality for the GPS reception can be used to obtain the GPS-based location-determination benefits. The signal processing and analysis involved for the GPS signals includes added facilities to receive the GPS frequency band signals with their particular signal formats. The wireless unit must support the energy or power demands of this added functionality. In order to minimize the power drain in the mobile unit, the GPS reception may not be continuously active, e.g., when not needed for location support. However, acquisition and reception of the GPS signals requires a search for the applicable satellite signals at the time the location service is needed, and this search can result in a comparatively large time to first fix (TTFF) when the GPS receiver has not been actively monitoring the satellite signals for some time. Finally, the GPS receiver should be able to acquire and measure the characteristics of an adequate number of satellite signals across a relatively broad and uniform expanse of the sky in order to support the calculation of a location of acceptable accuracy. When the propagation paths from the satellites to the receiver are occluded or significantly distorted (e.g., by multipath propagation), a GPS-based solution is not available for the location determinations. Such signal occlusions and distortions persist when the GPS receiver is under foliage, behind terrain features, in the interiors of buildings, and/or at the base of downtown “urban canyons” with tall buildings obscuring the view of the sky.
Techniques that “assist” a GPS receiver to mitigate some of the above difficulties are described in the above-cited U.S. Pat. Nos. 4,445,118 and 6,538,600 B1. In supporting an assisted GPS (AGPS) receiver, the external AGPS infrastructure may be able to provide the guiding information that facilitates the reception of the necessary GPS signals when the strength of the signals is moderately degraded. Furthermore the assistance can result in the implementation of the GPS receiver with simpler, lower power circuitry. Perhaps most significantly, the assistance provided to the AGPS receiver guides the receiver in the parameters appropriate for acquiring those and only those satellites that are currently “over head.” Thus the assistance reduces the signal search processing required to detect the satellite signals and thereby supports enhanced performance with a reduced response TTFF.
Despite these enhancements that the AGPS approach provides, the difficulties presented for adequate GPS signal acquisition in “heavy” urban environments still significantly degrade or effectively prevent a GPS-based location to an unfortunate extent. The volume of wireless communications traffic in these environments makes these difficulties an unacceptable burden in supporting public safety or emergency responses and in providing the productivity enhancements that location-based services can facilitate.
Further background information concerning wireless location can be found in the following United States patents, which are owned by TruePosition, Inc., the assignee of the present invention: U.S. Pat. No. 6,661,379 B2, Dec. 9, 2003, Antenna Selection Method for a Wireless Location System; U.S. Pat. No. 6,646,604, Nov. 11, 2003, Automatic Synchronous Tuning Of Narrowband Receivers Of A Wireless Location System For Voice/Traffic Channel Tracking; U.S. Pat. No. 6,603,428, Aug. 5, 2003, Multiple Pass Location Processing; U.S. Pat. No. 6,563,460, May 13, 2003, Collision Recovery In A Wireless Location System; U.S. Pat. No. 6,519,465, Feb. 11, 2003, Modified Transmission Method For Improving Accuracy For E-911 Calls; U.S. Pat. No. 6,492,944, Dec. 10, 2002, Internal Calibration Method For Receiver System Of A Wireless Location System; U.S. Pat. No. 6,483,460, Nov. 19, 2002, Baseline Selection Method For Use In A Wireless Location System; U.S. Pat. No. 6,463,290, Oct. 8, 2002, Mobile-Assisted Network Based Techniques For Improving Accuracy Of Wireless Location System; U.S. Pat. No. 6,400,320, Jun. 4, 2002, Antenna Selection Method For A Wireless Location System; U.S. Pat. No. 6,388,618, May 14, 2002, Signal Collection System For A Wireless Location System; U.S. Pat. No. 6,351,235, Feb. 26, 2002, Method And System For Synchronizing Receiver Systems Of A Wireless Location System; U.S. Pat. No. 6,317,081, Nov. 13, 2001, Internal Calibration Method For Receiver System Of A Wireless Location System; U.S. Pat. No. 6,285,321, Sep. 4, 2001, Station Based Processing Method For A Wireless Location System; U.S. Pat. No. 6,334,059, Dec. 25, 2001, Modified Transmission Method For Improving Accuracy For E-911 Calls; U.S. Pat. No. 6,317,604, Nov. 13, 2001, Centralized Database System For A Wireless Location System; U.S. Pat. No. 6,281,834, Aug. 28, 2001, Calibration For Wireless Location System; U.S. Pat. No. 6,266,013, Jul. 24, 2001, Architecture For A Signal Collection System Of A Wireless Location System; U.S. Pat. No. 6,184,829, Feb. 6, 2001, Calibration For Wireless Location System; U.S. Pat. No. 6,172,644, Jan. 9, 2001, Emergency Location Method For A Wireless Location System; U.S. Pat. No. 6,115,599, Sep. 5, 2000, Directed Retry Method For Use In A Wireless Location System; U.S. Pat. No. 6,097,336, Aug. 1, 2000, Method For Improving The Accuracy Of A Wireless Location System; U.S. Pat. No. 6,091,362, Jul. 18, 2000, Bandwidth Synthesis For Wireless Location System; U.S. Pat. No. 5,608,410, Mar. 4, 1997, System For Locating A Source Of Bursty Transmissions; and U.S. Pat. No. 5,327,144, Jul. 5, 1994, Cellular Telephone Location System. Other exemplary patents include: U.S. Pat. No. 6,546,256 B1, Apr. 8, 2003, Robust, Efficient, Location-Related Measurement; U.S. Pat. No. 6,366,241, Apr. 2, 2002, Enhanced Determination Of Position-Dependent Signal Characteristics; U.S. Pat. No. 6,288,676, Sep. 11, 2001, Apparatus And Method For Single Station Communications Localization; U.S. Pat. No. 6,288,675, Sep. 11, 2001, Single Station Communications Localization System; U.S. Pat. No. 6,047,192, Apr. 4, 2000, Robust, Efficient, Localization System; U.S. Pat. No. 6,108,555, Aug. 22, 2000, Enhanced Time Difference Localization System; U.S. Pat. No. 6,101,178, Aug. 8, 2000, Pseudolite-Augmented GPS For Locating Wireless Telephones; U.S. Pat. No. 6,119,013, Sep. 12, 2000, Enhanced Time-Difference Localization System; U.S. Pat. No. 6,127,975, Oct. 3, 2000, Single Station Communications Localization System; U.S. Pat. No. 5,959,580, Sep. 28, 1999, Communications Localization System; and U.S. Pat. No. 4,728,959, Mar. 1, 1988, Direction Finding Localization System.
In sum, over the past ten years organizations within the wireless telecommunications industry have invested significant time and resources in studying wireless location technologies. Of the technologies investigated to date, all have proven to have certain strengths and weaknesses. As of yet no single location technology has been identified that provides optimal performance across all environments. As a result, it is desirable to have a set of complementary technologies that together can provide acceptable performance across all reasonable circumstances. For example, in significant live field deployments to date, location technologies based on uplink time difference of arrival (U-TDOA) techniques have proven to provide excellent performance in urban, suburban and indoor environments. The U-TDOA technologies do not require modifications to handsets, and so performance for existing mobile stations has proven to be excellent in these same environments. In some rural environments, where cell site densities, network geometries, and coverage areas are very limited, the performance of U-TDOA has proven to degrade without the assistance of other location methods. The Assisted Global Position Systems (AGPS) and Advanced Forward Link Trilateration (AFLT) location technologies also have significant location capabilities, but they also have weaknesses. For example, in urban and indoor environments where reception of GPS signals becomes very difficult and sometimes impossible, the performance of AGPS technologies both in accuracy and yield degrades significantly. However, in these same urban and indoor environments U-TDOA technologies have proven to perform well because the SNR of uplink channels remains high and cell site densities are most dense. Additionally, in urban and dense suburban environments, where higher accuracies become more valuable to the carrier and subscriber but the effects of multipath become more significant, the performance of AFLT technologies is limited by their inability to mitigate the effects of multipath. (See presentation, GPS-Assisted Location Technology, Alpha Trial Field Test in Tampa, Fla., Mar. 9-Apr. 2, 1999, a copy of which is being submitted herewith in an Information Disclosure Statement.) In these same urban and dense suburban environments U-TDOA technologies have proven to perform well due to their ability to utilize advanced super-resolution techniques to mitigate the effects of multipath. Finally, U-TDOA is able to cover 100% of existing mobile stations today, including the new AGPS and AFLT capable mobiles. The AGPS and AFLT location methods, however, depend on subscribers purchasing new location capable mobile stations from a limited set of vendors.